1. Field of the Invention
This invention relates to an ophthalmic measurement apparatus, and more particularly to an ophthalmic measurement apparatus in which a laser beam is projected at a selected spot in an eye under examination and laser light scattered therefrom is detected for ophthalmic measurement.
2. Description of the Prior Art
Measurement of protein concentration (the flare components) in the aqueous chamber of the anterior camera oculi is of considerable importance in determining whether the camera oculi is inflamed, that is, whether the blood-aqueous barrier function is normal or not. A number of methods are employed for carrying out this measurement. One method frequently used employs a slit lamp microscope to grade the concentration by naked-eye observation, while a photographic measuring method has been developed as a way of measuring the protein concentration quantitatively. In another method, a beam of laser light is directed into the eye and the light scattered from the eye is detected and quantitatively analyzed. With the method employing laser light, a laser beam from a laser light source in a projection section is converged at a selected spot in the camera oculi of the eye together with a slit image, thereby providing illumination around the spot so that, via an observation section, the projected laser beam and slit image can be observed. To carry out the ophthalmic measurement, the scattered light from the eye is detected by a photodetector which converts the light into electrical signals that are then processed.
In this type of ophthalmic measurement apparatus, a beam splitter or semi-transparent mirror is used to divide the scattered light from the eye into light that is guided to the photodetector and light that is guided to the observation section (see Japanese Patent Publication 63-55928 and Japanese Laid-open Patent Application 63-315030 corresponding to U.S. patent application Ser. No. 206,518 filed on June 14, 1988). As the beam splitter or semi-transparent mirror usually reduces the amount of light by half, the amount of light available for observation purposes also is reduced. If I1 is the amount of incident light reaching the observation section, I2 is the amount of incident light at the photodetector and the transmittance of the beam splitter or semi-transparent mirror is 50%, then I1=I2 =1/2, thus I1 is half the amount it would be if there were no beam splitter or semi-transparent mirror. This is a major drawback when a slit lamp microscope is used. One solution is to use a lens with a large aperture, but the resulting increase in the cost and size of the apparatus makes this impractical. On the other hand, the increase in the amount of light to the observation section decreases the amount of incident light at the photodetector, thus impeding the measurement of flare components.
In Japanese Laid-open Patent Application 63-315030, an arrangement is used in which P-polarized light impinges on a semi-transparent mirror, but the efficiency of this is low. To improve the efficiency, a.lambda./2 plate is provided in front of the semi-transparent mirror and the plane of polarization is rotated 90 degrees so that S-polarized light impinges on the semi-transparent mirror, increasing the amount of light received by the light receiving section. By changing the scattered light impinging on the semi-transparent mirror into S-polarized light, the amount of light impinging on the photodetector can be increased without affecting the amount of light received by the observation section. However, a.lambda./2 plate is expensive and raises the overall cost of the apparatus, while another disadvantage is that the apparatus cannot be used as a slit lamp microscope.